For decades physicists were convinced that gravity should be causing the expansion rate of the universe to slow.

"When I throw my keys up in the air, the gravity of the Earth makes them slow down and return to me," said Mario Livio, a theoretical physicist at the Space Telescope Science Institute (STScI) in Baltimore, Maryland.

But the study, along with an independent work released later the same year, showed that the expansion rate is actually speeding up.

This observation, Livio said, is as if "the keys suddenly went straight up toward the ceiling."

Scientists attribute the phenomenon to dark energy, a force that repels gravity. Even more surprising, measurements show that dark energy accounts for about 74 percent of the substance of the universe.

A decade later, a new suite of experiments may pin down the properties of dark energy and solve what some experts are calling "the most profound problem" in modern physics.

"This is game-changing science," Michael Turner, a cosmologist at the University of Chicago, told a packed auditorium during the Decade of Dark Energy Symposium held last week at STScI.

"We've gone from establishing the phenomenon to probing the underlying cause," he said. "We're not anywhere near the point where it's time to give dark energy a rest."

Energy in a Vacuum

So far, one of the biggest challenges for dark energy researchers is marrying observations to theory.